Micromechanical actuator with multiple-plane comb electrodes and methods of making

ABSTRACT

A micro-electro-mechanical component comprising a movable element with comb electrodes, and two stationary elements with comb electrodes aligned and stacked on each other but electrically insulated by a layer of insulation material. The movable element is supported by multiple torsional hinges and suspended over a cavity such that the element can oscillate about an axis defined by the hinges. The comb electrodes of the movable element are interdigitated with the comb electrodes of one stationary element in the same plane to form an in-plane comb actuator. The comb electrodes of the movable element are also interdigitated in an elevated plane with the comb electrodes of another stationary element to form a vertical comb actuator. As a result, the micro-electro-mechanical component is both an in-plane actuator and a vertical comb actuator, or a multiple-plane actuator. Methods of fabricating such actuator are also described.

BACKGROUND OF THE INVENTION

The present invention relates to a micro-electro-mechanical component,and more particularly to actuator.

Micro-electro-mechanical mirrors have great potential in wide variety ofoptical applications including optical communication, confocalmicroscope, laser radar, bar code scanning, laser printing andprojection display. For some optical scanning applications such as laserprinting and scanning projection display, the mirror needs to achievelarge optical scanning angle at specific frequency. Large optical angleis also a key to optical resolution and smaller product footprint. Forscanning mirror, this requirement poses a challenge in the design ofactuator to generate large actuation force. A variety ofmicro-electro-mechanical actuator designs have been proposed to steer orscan light beam for various applications. In order to achieve deflectionor movement of the micro-component out of the chip plane, it is known todesign a movable element containing electrodes and a stationary elementcontaining counter-electrodes such that the movable element can bedriven by the electrical force.

In U.S. Pat. No. 6,595,055, Harald Schenk, et al described amicromechanical component with both the oscillating body and the frameor stationary layer located on the same chip plane. Capacitance isformed between the lateral surfaces of the oscillating body and theframe layer and will vary as the movable body oscillates about a pivotaxis out of the chip plane. The structure is suspended and supported byan insulating layer and a substrate to allow out-of-plane motion of theoscillating body. They described in “Large Deflection MicromechanicalScanning Mirrors for Linear Scan and Pattern Generation” in Journal ofSelected Topics in Quantum Electronics, Vol 6, No 5, 2000 that thescanning mirror can scan at large angle with low driving voltage at lowfrequency. However, movable comb electrodes located on the mirrorperimeter will increase dynamic deformation of the mirror or movablebody. Excessive dynamic deformation of scanning mirror will increasedivergence of reflected light beam and significantly deteriorate opticalresolution of the device for high speed scanning applications such asprinting and scanned display. Additional electrode insulated from thestructure may be required to perturb the symmetry of the setup in orderto quickly initiate oscillation of the mirror. Furthermore, the setuponly allows analog operation (scanning) but not digital operation(static angle positioning) of the movable body.

R. Conant describes in “Staggered Torsional Electrostatic Combdrive andMethod of Forming SAME” (Patent Application U.S. 2003/0019832), acomb-drive actuator with a stationary comb teeth assembly and a movingcomb teeth assembly with a mirror and a torsional hinge, and the methodof fabricating such devices. The moving assembly is positioned entirelyabove the stationary assembly by a predetermined vertical displacementduring resting state. The actuator is able to scan at relative highfrequency with mirror dynamic deformation lower than the Rayleigh limit.However, the optical scan angle which dominates the optical resolutionis notably smaller than what Schenk has reported despite a relative highvoltage is applied. An alternate design was proposed with additionalstationary comb teeth assemblies stacked on top of the stationary combteeth assembly. This stacked comb teeth assemblies were claimed to beused for the purpose of capacitive sensing and frequency tuning of themovable assembly despite that the method of frequency tuning was notdescribed. In the fabrication process steps, a process step is requiredto open alignment windows by etching through the top wafer to reach theinsulating oxide layer then removing the oxide layer in order to usefeatures located on the bottom wafer for alignment of subsequent steps.If the top wafer is thick for the purpose of minimizing dynamicdeformation, this process could be time-consuming and hence, expensive.

S. Olav describes in “Self-Aligned Vertical Combdrive Actuator andMethod of Fabrication” (US Patent Application U.S. 2003/0073261), avertical comb-drive actuator with small gaps between comb teeth forincreased torsional deflection, a double-sided vertical comb-driveactuator for dual-mode actuation, vertical piston and scan, and themethod of making them. Despite the proposed fabrication process stepsallow self-alignment of the embedded comb teeth, the process of verticalcomb-drive actuator requires highly skilled techniques to etch thebottom comb teeth and twice deep silicon trench etching of the bottomsubstrate. For dual-mode vertical comb-drive actuator, the fabricationprocess steps start with deep silicon trench etching of the device layerof a Silicon-On-Insulator (SOI) wafer then fusion bonding to anothersilicon wafer that resulting in a complex five-layer structure, twoinsulation oxide layers and three silicon layers. To form the bottomcomb teeth highly skilled self-alignment etching techniques and twicedeep silicon trench etching are still required.

SUMMARY OF THE INVENTION

It is the objective of the present invention to provide amicro-electro-mechanical actuator with in-plane comb electrodes and asupporting substrate with a cavity of specific depth.

It is the objective of the present invention to provide amicro-electro-mechanical actuator with both in-plane and vertical combelectrodes that increase the actuation force on the movable element, andthe methods of fabricating such device.

It is a further objective of this invention to provide amicro-electro-mechanical actuator with both in-plane and dual-sidevertical comb electrodes that increase the actuation force on themovable element, and the methods of fabricating such devices.

It is another objective of this invention to provide a method to supportand fan out the bottom electrodes of the vertical comb electrodes.

It is another objective of this invention to provide a torsional hingedesign with built-in electrodes that can be used to increase theeffective torsional stiffness of the hinges such that the resonancefrequency of the movable element in an actuator can be adjusted.

It is another objective of this invention to provide a method todecrease the effective torsional stiffness of the torsional hinges suchthat the resonance frequency of the movable element in an actuator canbe adjusted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C show the top views of the top, middle and bottomlayers of one embodiment of the present invention.

FIG. 1D, 1E and 1F illustrate the fabrication process flow steps of theembodiment described in FIGS. 1A, 1B and 1C.

FIGS. 1G and 1H illustrate another fabrication process flow steps of theembodiment described in FIGS. 1A, 1B and 1C.

FIG. 2A˜2D illustrate another side view of the embodiment described inFIG. 1 and show the relationship of actuation force of in-plane andvertical comb electrodes when the mobile element of top layer is inoscillation motion. The vertical comb electrodes on the bottom layer arelocated only on one side of the torsional hinges.

FIG. 3 illustrates one example of the relationship between the phase ofmirror deflection angle and the phase of applied voltage sources forMEMS actuator depicted in FIG. 2.

FIG. 4 illustrates the three dimensional view of the present inventionwhere the mobile element is supported by a pair of torsional hinges andactuated by both in-plane and vertical comb structure.

FIGS. 5A, 5B, and 5C show the top views of the top, middle and bottomlayers of another embodiment of present invention where vertical combelectrodes on the bottom layer are electrically isolated into two halvesof the different sides of the torsional hinges. Three voltage sourcescan be applied to achieve large actuation force on the mobile element.

FIG. 5D illustrates another design of the bottom layer of the embodimentas depicted in FIG. 5C. The two sets of electrically isolated verticalcomb electrodes are reinforced through thin film deposition processes.

FIG. 6A˜6D illustrate one fabrication process flow steps of theembodiment as described in FIGS. 5A, 5B and 5C.

FIG. 7A˜7F illustrate the fabrication process flow steps of theembodiment as described in FIG. 5A, 5B and 5D.

FIG. 8A˜8D illustrate the side view of the embodiment as described inFIG. 5 and show the relationship of actuation force of in-plane andvertical comb electrodes when the mobile structure of top layer is inoscillation motion. The vertical comb electrodes on the bottom layer areelectrically isolated on each side of the torsional hinges.

FIG. 9 illustrates one example of the relationship between the phase ofmirror deflection angle and the phase of applied voltage sources forMEMS actuator depicted in FIG. 8.

FIG. 10A, 10B, and 10C illustrate the methods to connect the two set ofelectrically isolated vertical comb electrodes located on the bottomlayer of the actuator as described in FIG. 1 and FIG. 5.

FIG. 11 illustrates another embodiment of the invention that additionalin-plane comb electrodes are added to the torsional hinges and to thestationary structure on the top layer of the actuator. A voltagedifference between the additional comb electrodes sets may be applied toincrease the effective stiffness of the hinges.

FIG. 12 illustrates the torsional hinge with protrusion areas that maybe removed by laser or other means to reduce the torsional stiffness ofthe hinge.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A, 1B and 1C show the exploded top views of the three layers of aMEMS actuator in accordance with one embodiment of the presentinvention. Top layer, FIG. 1A, is consisted of a stationary and amovable elements both made of electrically conductive material,typically doped single crystal silicon. Movable element including combelectrodes is supported by multiple torsional hinges and is electricallyisolated from stationary structure. The stationary element has combelectrodes that are interdigitated in the same horizontal plane with thecomb electrodes of the movable element such that the top layer is anin-plane comb-drive actuator. Middle layer, FIG. 1B, is made ofelectrically non-conductive material, typically silicon dioxide. Bottomlayer, FIG. 1C, consisting of a cavity and stationary comb electrodeslocated on one side of the torsional hinge, is made of electricallyconductive material, typically doped single crystal silicon. Stationarycomb electrodes on bottom layer are interdigitated with comb electrodesof the movable element on top layer such that the movable element andthe bottom layer form a vertical comb-drive actuator. Middle and bottomlayers support the top layer while middle layer electrically isolatestop and bottom layers. As a result, the MEMS actuator is consisted ofboth in-plane and vertical comb-drive actuators.

The movable element is typically connected to electrical ground whilethe stationary element on the top layer is connected to a voltage sourceand the bottom layer is connected to another voltage source. FIG. 3illustrates the phase and amplitude relationships between deflectionangle of movable element and applied voltage sources. The waveform ofthe voltage source can be square, triangular, sinusoidal,half-sinusoidal or other shapes to meet specific angular velocity needs.

FIG. 1D-1F illustrate one method of fabricating the comb-drive actuatorin accordance with one embodiment of the present invention as describedin FIG. 1A-1C. The first step, FIG. 1D, starts by etching the backsideof a semiconductor wafer, preferably single crystal silicon then etchesthe front-side using deep reactive ion etching (DRIE) with the etchedfeatures on backside for alignment. The next step is to fusion bond thedouble-side etched wafer to another wafer coated with silicon dioxidethen annealed to increase bonding strength. The bonded wafer becomes athree layer structure and the top layer may be ground and polished todesired thickness and to the required surface quality, FIG. 1E. The toplayer is then DRIE etched down to the middle layer using the backsidefeatures for alignment and the movable element of the three-layerstructure is released by removing the silicon dioxide connecting to thestationary elements, FIG. 1F.

FIGS. 1G and 1H illustrate another fabrication method of the comb-driveactuator. The process starts with back-side DRIE etching to the middleoxide layer of a silicon-on-insulator (SOI) wafer, FIG. 1G. The wafer isthen etched from the front-side of the wafer to the middle oxide layer,FIG. 1H. The movable element of the three-layer structure is thenreleased by removing the silicon dioxide connecting to the stationaryelements.

FIG. 2A-2D and FIG. 3 show the operation of the MEMS actuator asdescribed in FIG. 1. The movable element is connected to electricalground, the top stationary comb electrodes and the bottom stationarycomb electrodes are connected to the first and the second AC voltagesources, respectively as shown in FIG. 2A. Top stationary and movableelements form an in-plane comb actuator whereas bottom stationaryelement and top movable element form a vertical comb actuator. Themovable element starts oscillation with respect to the torsional hingesthrough either the unbalance of electrostatic force in the in-plane combactuator or the electrostatic attraction from the vertical actuator,FIG. 2A. The unbalance force in the in-plane comb may be introduced frommanufacturing tolerances or intentional design features. Electrostaticattraction force from the vertical comb actuator will rotate the movableelement with respect to the torsional hinges to the maximum deflectionangle, FIG. 2A˜2B. After the movable element reaches the largestdeflection angle, electrostatic attraction force from the in-plane combactuator will be applied to the movable element until horizontalposition is restored, FIG. 2B˜2C. The movable element continues torotate without actuation force to another maximum deflection angle, FIG.2C˜2D. After the movable element reaches another maximum deflectionangle, electrostatic attraction force from the in-plane comb actuatorwill again be applied to the movable element until horizontal positionis restored to complete one oscillation cycle, FIG. 2D˜2A.

FIG. 3 illustrates the relationship of the applied voltage sources andthe operation of the MEMS actuator corresponding to FIG. 2. The movableelement is typically designed to oscillate at or near its resonancefrequency of primary oscillation mode. The movable element including topmovable comb electrodes is connected electrical ground. The firstvoltage source AC1 is applied to the top stationary structure within-plane comb electrodes. The second voltage source AC2 is applied tothe bottom stationary comb electrodes. The frequency of voltage sourceAC1 is typically twice the oscillation frequency of the movable element.The frequency of voltage source AC2 is the same as the oscillationfrequency of the movable element. The waveform of AC1 and AC2 can bevarious shapes to achieve desired angular velocity of the movableelement. Typically, waveform of square shape gives the highestefficiency in driving the movable element to the largest rotation angleunder given amplitude of AC1 and AC2. FIG. 4 shows a three-dimensionalview of the MEMS actuator with movable element rotating to its largestangle.

The present invention combines both in-plane and vertical comb actuatorsto drive the movable element to oscillate at large angle and at highfrequency. Furthermore, the cavity depth in the bottom layer of theactuator, described in fabrication flow of FIG. 1D, 1E and 1F, can bedesigned to be a mechanical stop to prevent excess deflection of themovable structure that could induce mechanical failure of the actuator.

FIGS. 5A, 5B and 5C show the exploded top views of the three layers of aMEMS actuator in accordance with another embodiment of the presentinvention. Top layer, FIG. 5A, is consisted of a stationary and amovable elements, both made of electrically conductive material,typically doped single crystal silicon. Movable element including combelectrodes is supported by multiple torsional hinges and is electricallyisolated from stationary structure. The stationary element has combelectrodes that are interdigitated in the same horizontal plane with thecomb electrodes of the movable element such that the top layer is anin-plane comb-drive actuator. Middle layer, FIG. 5B, is made ofelectrically non-conductive material, typically silicon dioxide. Bottomlayer, FIG. 5C, consisting of a cavity and stationary comb electrodes,is made of electrically conductive material, typically doped singlecrystal silicon. Comb electrodes on the bottom layer are electricallyisolated into two halves located on different sides of the torsionalhinges. Stationary comb electrodes on bottom layer are interdigitatedwith comb electrodes of the movable element on top layer such that themovable element and the bottom layer form a vertical comb-drive actuatorwith dual-side driving capability. Middle and bottom layers support thetop layer while middle layer electrically isolates top and bottomlayers. As a result, the MEMS actuator is consisted of both in-plane andvertical comb-drive actuators.

FIG. 6A-6D illustrate one method of fabricating the comb-drive actuatorin accordance with the embodiment as described in FIG. 5A-5C. The firststep, FIG. 6A, starts by etching the backside of a semiconductor wafer,preferably single crystal silicon then etches the front-side using deepreactive ion etching (DRIE) with the etched features on backside foralignment. Cavity size and depth, and the stationary vertical combelectrodes are defined. The next step is to fusion bond the double-sideetched wafer to another wafer coated with silicon dioxide then annealedto increase bonding strength, FIG. 6B. The bonded wafer becomes a threelayer structure and the top layer may be ground and polished to desiredthickness and to the required surface quality. Backside of the bondedwafer is separated into two halves using DRIE, FIG. 6C. Since the bottomlayer is bonded to the top layer so the three layer structure remainsintact. The top layer is then DRIE etched down to the middle layer usingthe backside features for alignment and the movable element of thethree-layer structure is released by removing the silicon dioxideconnecting to the stationary elements, FIG. 6D.

The comb-drive actuator, described in FIGS. 5A, 5B and 5C, can also befabricated using process flow steps of FIG. 1G and 1H. The processstarts with back-side DRIE etching of the bottom layer to the middleoxide layer of a SOI wafer and also separates the bottom layer into twohalves, FIG. 1G. Since the bottom layer is bonded to the top layer sothe three layer structure remains intact. The wafer is then etched fromthe front-side of the wafer to the middle oxide layer, FIG. 1H. Themovable element of the three-layer structure is then released byremoving the silicon dioxide connecting to the stationary elements.

FIG. 5D shows a variation of the bottom layer as described in FIG. 5C.The bottom layer are electrically isolated into two halves andreinforced with thin film deposited materials. The reinforcementmaterials must have electrically non-conductive materials such assilicon dioxide. The comb-drive actuator, defined by FIGS. 5A, 5B and5D, can be fabricated with process steps of FIG. 7A˜7F. Process steps ofFIG. 7A˜7C is the same as process steps of FIG. 6A˜6C. After thebackside of wafer is etched and separated into two halves, FIG. 7C,electrically isolated material such as silicon dioxide is deposited onthe backside and the opened channels using thin film processes, FIG. 7D.Another layer of material, such as polysilicon, is further deposited onthe backside and the opened channels to complete the reinforcement, FIG.7E. The thin film materials on the backside may be removed by grindingand polishing. Top layer is then DRIE etched down to the middle layerusing the backside features for alignment and the movable element of thethree-layer structure is released by removing the silicon dioxideconnecting to the stationary elements, FIG. 7F.

FIG. 8 and FIG. 9 illustrate the operation of the MEMS actuator asdescribed in FIG. 5. Movable element on top layer is connected toelectrical ground while stationary comb electrodes is connected thefirst AC voltage source. The two sets of bottom stationary combelectrodes are connected to the second and the third AC voltage sources,respectively as shown in FIG. 8A. Movable element starts oscillationwith respect to the torsional hinges through either the unbalance ofelectrostatic force in the in-plane comb electrodes or the electrostaticattraction from the vertical comb electrodes, FIG. 8A. The unbalanceforce in the in-plane comb may be introduced from manufacturingtolerances or intentional design features. Electrostatic attractionforce from one side of the vertical comb actuator will rotate themovable element with respect to the torsional hinges to the maximumdeflection angle, FIG. 8A˜8B. After the movable element reaches thelargest deflection angle, electrostatic attraction force from thein-plane comb actuator will be applied to the movable element untilhorizontal position is restored, FIG. 8B˜8C. Electrostatic attractionforce from another side of the vertical comb electrodes will rotate themovable element to another maximum deflection angle, FIG. 8C˜8D. Afterthe movable element reaches another maximum deflection angle,electrostatic attraction force from the in-plane comb actuator willagain be applied to the movable element until horizontal position isrestored to complete one oscillation cycle, FIG. 8D˜8A.

FIG. 9 illustrates the relationship of the applied voltage sources andthe operation of the MEMS actuator corresponding to FIG. 5. The movableelement is typically designed to oscillate at or near its resonancefrequency of primary oscillation mode. The movable element including topmovable comb electrodes is connected electrical ground. First voltagesource AC1 is applied to the top stationary structure with in-plane combelectrodes. Second voltage source AC2 is applied to one set of thebottom stationary comb electrodes. Third voltage source AC3 is appliedto another set of the bottom stationary comb electrodes. The frequencyof voltage source AC1 is typically twice the oscillation frequency ofthe movable element. The frequency of voltage sources AC2 and AC3 arethe same as the oscillation frequency of the movable element but atdifferent phases. The waveform of AC1, AC2 and AC3 can be various shapesto achieve desired angular velocity of the movable element. Typically,waveform of square shape gives the highest efficiency in driving themovable element to the largest rotation angle under given amplitude ofAC1, AC2 and AC3.

FIG. 10A illustrates a method to form electrical connections to thebottom layer of the actuator with SOI structure. Additional openings onthe top layer are etched in DRIE etching process step as described inFIG. 1F, 1H, 6D or 7F to expose access to the middle layer. Electricalinsulation material of the middle layer in the exposed area is thenremoved during structure release process. Connections can be made to thebottom layer through conventional methods such as wire-bonding afterdeposition of metallic contact pad.

FIGS. 10B and 10C illustrate another method to form electricalconnections to the bottom layer of the actuator with SOI structure. TheSOI structure is connected to a substrate through a layer ofelectrically conductive material which is separated into two halves toavoiding electrical bridging. The conductive material may be conductivepaste, conductive film, solder paste, etc. The substrate is configuredfor fan-out of the bottom comb electrodes. Dielectric material isdisposed on the substrate which insulates the metal conductor pads onthe substrate. Fan-out can be done on the from the top side conductorpads of the substrate, FIG. 11B or from bottom side conductor padsconnecting to top side conductor pads through via holes, FIG. 11C.

FIG. 11 illustrates one invention embodiment to adjust the structuralresonance frequency of the movable element by increasing the effectivetorsional stiffness of the torsional hinges. Torsional hinges aredesigned with comb electrodes and are interdigitated with a set of combelectrodes on the stationary structure of the top layer. This set ofcomb electrodes on the top stationary structure are connected to a DCvoltage source and are electrically isolated from the rest of the combelectrodes on the top layer. During oscillation motion of the movableelement, the voltage difference between the DC voltage and the groundwill generate electrostatic attraction force between the additional combelectrodes which will suppress the torsional rotation of the portion ofhinge with additional electrodes. By adjusting the voltage differencebetween DC and ground, the effective torsional stiffness of the hingescan be increased such that resonance frequency of the movable elementcan be tuned.

FIG. 12 illustrates another invention embodiment to adjust thestructural resonance frequency of the movable element by thinningportions or trimming portions of protrusions on the torsional hinges.The protrusions may be removed selectively utilizing techniques such aslaser trimming, E-beam lithography, etc without damaging structuralintegrity. The effective torsional stiffness of the torsional hinges arereduced such that the resonance frequency of the movable element can betuned.

1. A micro-electro-mechanical comb-drive actuator, comprising: asemiconductor layer consisting a movable element and a stationaryelement where the movable element is supported by multiple hinges toallow rotation about an axis and has two sets of comb electrodes locatedon opposite sides of the axis, and the stationary element, electricallyisolated from the movable element, has comb electrodes interdigitatedwith the comb electrodes of the movable element; an electricallyinsulation layer which supports and insulates the semiconductor layers;a second semiconductor layer consisting a cavity.
 2. Amicro-electro-mechanical comb-drive actuator according to claim 1, thecavity of the second semiconductor layer has specific depth asmechanical stop of the movable element to prevent failure of torsionalhinges due to over rotation.
 3. A micro-electro-mechanical comb-driveactuator according to claim 1, the second semiconductor contains one setof stationary comb electrodes located on one side of the rotation axisof the movable element and interdigitated with one set of combelectrodes of the movable element.
 4. A micro-electro-mechanicalcomb-drive actuator according to claim 1, the movable element isconnected to electrical ground, the stationary element of the firstsemiconductor layer is connected to the first AC voltage source, and thestationary comb electrodes of the second semiconductor layer areconnected to the second AC voltage source.
 5. A micro-electro-mechanicalcomb-drive actuator according to claim 1, wherein the firstsemiconductor layer has an additional stationary element with combelectrodes that is electrically isolated from the movable and thestationary element, and the torsional hinges have comb electrodes thatare interdigitated with the comb electrodes of the additional stationaryelement.
 6. A micro-electro-mechanical comb-drive actuator according toclaim 5, wherein the second stationary element is connected to a DCvoltage source to tune the resonance frequency of the movable element.7. A micro-electro-mechanical comb-drive actuator according to claim 1,wherein the hinges contain protrusions that can be removed or thinned totune the resonance frequency of the movable element.
 8. Amicro-electro-mechanical comb-drive actuator, comprising: asemiconductor layer consisting a movable element and a stationaryelement where the movable element is supported by multiple hinges toallow rotation about an axis and has two sets of comb electrodes locatedon opposite sides of the axis, and the stationary element, electricallyisolated from the movable element, has comb electrodes interdigitatedwith the comb electrodes of the movable element; an electricallyinsulation layer which supports and insulates the semiconductor layers;a second semiconductor layer consisting a cavity and two sets ofelectrically isolated stationary comb electrodes that are located oneach side of the rotation axis and are interdigitated with the combelectrodes of the movable element.
 9. A micro-electro-mechanicalcomb-drive actuator according to claim 8, the cavity of the secondsemiconductor layer has specific depth as mechanical stop of the movableelement to prevent failure of torsional hinges due to over rotation 10.A micro-electro-mechanical comb-drive actuator according to claim 8, themovable element is connected to electrical ground, the stationaryelement of the first semiconductor layer is connected to the first ACvoltage source and the two sets of stationary comb electrodes of thesecond semiconductor layer are connected to the second and the third ACvoltage sources, respectively.
 11. A micro-electro-mechanical comb-driveactuator according to claim 8, wherein the first semiconductor layer hasan additional stationary element with comb electrodes that iselectrically isolated from the movable and the stationary element, andthe torsional hinges has comb electrodes that are interdigitated withthe comb electrodes of the additional stationary element.
 12. Amicro-electro-mechanical comb-drive actuator according to claim 11,wherein the additional stationary element on the first semiconductorlayer is connected to a DC voltage source to tune the resonancefrequency of the movable element.
 13. A micro-electro-mechanicalcomb-drive actuator according to claim 8, wherein the hinges containprotrusions that can be removed or thinned to tune the resonancefrequency of the movable element.
 14. A method of fabricating thecomb-drive actuator comprising the steps of: etching back side of thefirst semiconductor wafer to define features for alignment of subsequentsteps; etching front side of the first semiconductor wafer to specificdepth to form a cavity and stationary comb electrodes by aligning tofeatures on the back side of wafer; oxidizing at least one surface of asecond semiconductor wafer; fusion bonding the first wafer with thesecond wafer with oxidized surface of the second wafer facing the etchedcavity and comb electrodes of the first wafers; etching the second waferto form the movable element and stationary element by aligning tofeatures on the back side of the first wafer such that the combelectrodes of the movable element are interdigitated with the combelectrodes of the stationary element which overlap the stationary combelectrodes of the first wafer but are insulated by the oxide layer. 15.A method of fabricating the comb-drive actuator comprising the steps of:etching back side of a Silicon-on-Insulator (SOI) wafer to form cavity,stationary comb electrodes and features for alignment of subsequentsteps; etching front side of the SOI wafer to form the movable elementand stationary element by aligning to features on the back side suchthat the comb electrodes of the movable element are interdigitated withthe comb electrodes of the stationary element which overlap thestationary comb electrodes of on the back side but are insulated by theoxide layer.
 16. A method of fabricating the comb-drive actuatorcomprising the steps of: etching back side of the first semiconductorwafer to define the features for alignment of subsequent steps; etchingfront side of the first semiconductor wafer to form cavity andstationary comb electrodes by aligning to features on the back side ofwafer; oxidizing at least one surface of a second semiconductor wafer;fusion bonding the first wafer with the second wafer with oxidizedsurface of the second wafer facing the etched cavity and comb electrodesof the first wafers; etching trenches on the backside of the first waferto electrically isolate the first wafer into two halves; etching thesecond wafer to form the movable element and stationary element byaligning to features on the back side of the first wafer such that thecomb electrodes of the movable element are interdigitated with the combelectrodes of the stationary element which overlap the stationary combelectrodes of the first wafer but are insulated by the oxide layer. 17.A method of fabricating the comb-drive actuator comprising the steps of:etching back side of the first semiconductor wafer to define thefeatures for alignment of subsequent steps; etching front side of thefirst semiconductor wafer to form cavity and stationary comb electrodesby aligning to features on the back side of wafer; oxidizing at leastone surface of a second semiconductor wafer; fusion bonding the firstwafer with the second wafer with oxidized surface of the second waferfacing the etched cavity and comb electrodes of the first wafers;etching trenches on the backside of the first wafer to electricallyisolate the first wafer into two halves; depositing a protective layer,eg silicon nitride of the backside of the first wafer; depositing orgrowing oxide on the trenches that electrically isolate the first waferinto two halves; depositing thin film material, such as polysilicon tothe trench and the back side of the first wafer such that the two halvesof the first wafer are connected but electrically insulated by theoxide; removing the polysilicon, oxide and the protective layer on thebackside of the first wafer; etching the second wafer to form themovable element and stationary element by aligning to features on theback side of the first wafer such that the comb electrodes of themovable element are interdigitated with the comb electrodes of thestationary element which overlap the stationary comb electrodes of thefirst wafer but are insulated by the oxide layer.
 18. Amicro-electro-mechanical component, comprising: a substrate with twoelectrically insulated conductors which is at least partially exposed;an electrostatic actuator with SOI structure where the bottom layer isconsisted of two halves that are electrically insulated to each other; alayer of adhesive material that is electrically conductive and dividedinto two insulated halves; whereas the adhesive material connects theactuator to the substrate such that the each conductor on the substrateconnects electrically to each half of the bottom layer of the actuator.19. The substrate as described in claim 18, further comprising via holesthat connect the conductors to the other side of the substrate.